Detector configurations for optical metrology

ABSTRACT

An apparatus is disclosed for obtaining ellipsometric measurements from a sample. A probe beam is focused onto the sample to create a spread of angles of incidence. The beam is passed through a quarter waveplate retarder and a polarizer. The reflected beam is measured by a detector. In one preferred embodiment, the detector includes eight radially arranged segments, each segment generating an output which represents an integration of multiple angle of incidence. A processor manipulates the output from the various segments to derive ellipsometric information.

PRIORITY

This application is a continuation of U.S. Ser. No. 10/696,269, filedOct. 29, 2003, now U.S. Pat. No. 6,836,328 which is in turn acontinuation of U.S. Ser. No. 10/137,606, filed May 2, 2002, now U.S.Pat. No. 6,678,046, which claimed priority to Provisional ApplicationSer. No. 60/315,514, filed Aug. 28, 2001, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

There is ongoing interest in expanding and improving the measurement ofsemiconductor wafers. A number of optical metrology tools have beendeveloped for non-destructively evaluating the characteristics of thinfilms formed on semiconductors during the fabrication process. Morerecently, optical metrology systems have been proposed for analyzing thegeometry of small periodic structures (critical dimensions) onsemiconductors.

Typical optical tools include reflectometry (both single wavelength andspectroscopic) and ellipsometry (again, both single wavelength andspectroscopic.) In some metrology tools, these various techniques arecombined. See for example U.S. Pat. Nos. 6,278,519 and 5,608,526, thedisclosures of which are incorporated herein by reference.

Other metrology tools have been developed which rely on measurements atmultiple angles of incidence (both single wavelength and spectroscopic).One class of such systems have been commercialized by the Assigneeherein are capable of deriving information about multiple angles ofincidence simultaneously. In these systems, a strong lens (highnumerical aperture) is used to focus a probe beam of light onto thesample in a manner to create a spread of angles of incidence. An arraydetector is used to measure the reflected rays of the probe beam as afunction of the position within the probe beam. The position of the rayswithin the probe beam corresponds to specific angles of incidence on thesample. Theses systems are disclosed in U.S. Pat. Nos. 4,99,014 and5,042,951, incorporated herein by reference. U.S. Pat. No. 4,99,014related to reflectometry while U.S. Pat. No. 5,042,951 relates toellipsometry. (See also, U.S. Pat. No. 5,166,752, also incorporatedherein by reference).

In a variant on this system, U.S. Pat. No. 5,181,080 (incorporated byreference), discloses a system in which a quad-cell detector (FIG. 1) isused to measure the reflected probe beam. Each quadrant 1–4 of thedetector measures an integration of all of the angles of incidencefalling on the sample. By subtracting the sums of opposite quadrants,ellipsometric information can be obtained. As described in the latterpatent, the information derived from the analysis corresponds to theellipsometric parameter delta δ which is very sensitive to the thicknessof very thin films on a sample.

The concepts of the latter patents were expanded to providespectroscopic measurements as described in U.S. Pat. No. 5,412,473, alsoincorporated herein by reference. In this patent, the system wasmodified to include a white light source. In one approach, a colorfilter wheel was used to sequentially obtain multiple wavelengthinformation. In another approach, a filter in the form of a rectangularaperture was used to select a portion of the reflected beam. Thisportion was then angularly dispersed onto an array with each rowproviding different wavelength information and each column containingthe various angle of incidence information.

U.S. Pat. No. 5,596,411, also incorporated by reference, disclosed apreferred approach for obtaining spectroscopic information for anintegrated multiple angle of incidence system of the type described inU.S. Pat. No. 5,181,080 discussed above. In this approach, a filter wasprovided that transmitted light along one axis and blocked light alongan orthogonal axis. The transmitted light was angularly dispersed andmeasured to provide spectroscopic information along one axis of theprobe beam. The filter was then rotated by ninety degrees to obtainmeasurements along the remaining axis. Various modifications of thisapproach were discussed, including splitting the beam and using twoidentical filters disposed orthogonal to each other to obtain bothmeasurements simultaneously. (See also “Characterization of titaniumnitride (TiN) films on various substrates using spectrophotometry, beamprofile reflectometry, beam profile ellipsometry and spectroscopic beamprofile ellipsometry,” Leng, et al., Thin Solid Films, Volume 313–314,1998, pages 309 to 313.)

The integrated multiple angle ellipsometric measurement system describedin U.S. Pat. No. 5,181,080, cited above has been successfullycommercialized and is incorporated into the Opti-Product sold by theAssignee herein. The technology is marketed under the trademark BeamProfile Ellipsometry. (See U.S. Pat. No. 6,278,519 cited above.) Asdescribed in the '080 patent, the four segments of the quad celldetector can be summed to provide information about the total reflectedpower of the probe beam. In addition, the sum of the output of thequadrants along one axis can be subtracted from the sum of the outputsof the remaining two quadrants to provide a result which is correspondsto the ellipsometric parameter δ.

This arrangement provides valuable information that can be used todetermine the thickness of thin films. However, the limited informationfrom this type of detection cannot typically be used to derive both ofthe ellipsometric parameters, Ψ and δ. U.S. Pat. No. 5,586,411,discloses that it would be possible to derive such information if one ofpolarizers were rotated and multiple measurements taken. As notedtherein at column 12, line 48, if enough measurements are taken, aFourier analysis can be performed on the data allowing the parameters ofΨ and δ to be extracted.

When designing commercial inspection systems, it is often desirable tominimize the number of moving parts. For example, moving parts oftencreate particulates that can contaminate the wafer. To the extent partsmust be moved, the motion systems must have high precision. Further,movements of parts that are specifically designed to modify opticalproperties, such as retarders or polarizers can effect how the systemtransmits and detects light.

Therefore, it is an object of the present invention to enhance theoperation of an integrated, simultaneous multiple angle ellipsometricsystem without the drawbacks of the prior approaches. In particular, thesubject invention is intended to permit the derivation of additionalellipsometric information, including both δ and Ψ. In one class ofembodiments, this additional information is derived in a system with animproved detector arrangement without the need for moving parts. Inanother class of embodiments, the rotating element is limited to thedetector which does not effect the polarization or retardation of thelight.

SUMMARY OF THE INVENTION

In a first embodiment, a narrowband light source such as a laser is usedto generate a probe beam. The polarized beam is focused onto the samplein a manner to create a large spread of angles of incidence. The probebeam light is passed through a quarter waveplate for retarding the phaseof one of the polarization states of the beam with respect to the otherpolarization state of the beam. A polarizer is provided for creatinginterference between the two polarization states in the reflected probebeam.

In accordance with the subject invention, a detector is provided witheight segments radially arranged around a center axis. In oneembodiment, eight pie shaped sections are provided in a configurationwhich is essentially a quad cell with each quadrant further divided inhalf. In another embodiment, the eight segments are arranged in anannular ring.

In either case, the output of the segments lying substantially along oneradial axis is subtracted from the output of the segments lyingsubstantially along an orthogonal radial axis. In order to gainadditional information, the output of the sectors lying along a thirdradial axis located midway between the first two orthogonal axes (i.e.at 45 degrees) is subtracted from the sectors lying along a fourthradial axis, perpendicular to the third axis. This extra informationobtained corresponds to an orientation essentially shifted by 45 degreesfrom the first measurements. When all the measurements are combined, thesample may be more accurately evaluated. This extra information can besupplied to conventional fitting algorithms to evaluate characteristicsincluding thin film thickness, index of refraction and extinctioncoefficient. In addition, geometrical parameters of structures formed onsemiconductors such as line width, spacing, and side wall angle andshape can also be evaluated. These calculations can be made using themeasurements directly. Alternatively, the measurements can be used toderive the ellipsometric parameters Ψ and Δ which are then used toevaluate the sample.

The use of an eight segment detector allows the extra measurement to beobtained simply by summing and subtracting outputs of the sectors in theprocessor. Other arrangements can be used to provide equivalent results.For example, the reflected beam could be split into two parts and thetwo quadrant detectors use, one offset by 45 degrees from the other.Alternatively, a single rotatable quadrant detector could be used. Afterthe first measurement is made, the detector could be rotated by 45degrees and a second measurement could be made. In a preferredembodiment, the output of all the segments is summed to provide ameasure of the full power of the reflected beam.

It may also be possible to rotate the either the polarizer or theretarder to achieve a similar result.

The concept can also be extended to the use of a two dimensionaldetector array. Using a processor, the elements on the array can becomputationally mapped to the eight segment and the analysis can be madeas described above. This approach can be particularly useful formeasurement of critical dimensions, where the orientation of the samplestructure and orientation of the probing radiation is significant andpossibly difficult to control.

The subject invention can also be extended to spectroscopicmeasurements. In this case, a white light source would typically be usedto generate a polychromatic probe beam. The probe beam could be passedthrough a color filter or monochrometer which sequentially transmitsnarrow bands of wavelengths. The filter or monochrometer wouldpreferably be located before the sample.

If simultaneous multiple wavelength information is desired, it isbelieved that the approach described in U.S. Pat. No. 5,596,411, whichincluded a rotating quadrant filter, grating and array detector would bemore suitable than the approach described herein.

Further objects of the subject invention can be understood withreference to the following detailed description, taken in conjunctionwith the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a quad-cell detector used in prior artmeasurements.

FIG. 2 is a schematic diagram of the optical lay-out of an apparatus forperforming the method of the subject invention.

FIG. 3 is a schematic diagram of an eight segment detector for use withthe subject invention.

FIG. 4 is a schematic diagram of a detector having eight detector armsaligned along four axes which can be used to implement the subjectinvention.

FIG. 5 is a schematic diagram of a detector having segments in the formof an annular ring which can be used to implement the subject invention.

FIGS. 6A and 6B are schematic diagrams of a two dimensional detectorarray which can be used to implement the subject invention.

FIGS. 7A and 7B are a schematic diagrams of a rotatable quadrantdetector which can be used to implement the subject invention.

FIG. 8 is a schematic diagram of a pair of quadrant detectors which canbe used to implement the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIG. 2, an apparatus 10 is illustrated for performing themethod of the subject invention. The apparatus lay out for thisembodiment is essentially the same as that described in U.S. Pat. No.5,181,080, except that the detector is configured with eight segments(FIG. 3) rather than four segments as in the prior art (FIG. 2). Theapparatus is designed to evaluate characteristics at the surface of asample 14, such as thin film layers 12 and/or structural features suchas critical dimensions.

In this embodiment, apparatus 10 includes a light source 20 forgenerating a probe beam 22 of radiation. One suitable light source is asolid state laser diode which emits a linearly polarized beam having astable, known and relatively narrow bandwidth. Probe beam 22 is turnedtowards the sample 14 with a 50/50 beam splitter 44. The probe beam isfocused onto the surface of the sample with a lens 26. In the preferredembodiment, lens 26 is defined by a spherical, microscope objective witha high numerical aperture on the order of 0.90 NA. The high numericalaperture functions to create a large spread of angles of incidence withrespect to the sample surface. The spot size is on the order of twentymicrons or less and is preferably five microns or less in diameter.

In should be noted that in this illustrated embodiment, the beam isdirected substantially normal to the surface of the sample prior tobeing focused by lens 26. This configuration helps minimize the spotsize on the sample. It is within the scope of the subject invention todirect the beam at a non-normal angle of incidence to the sample asshown in U.S. Pat. No. 5,166,752. Although using an off-axis beamincreases the spot size on the sample, high angles of incidence can becreated with a lower numerical aperture lens.

Turning back to FIG. 1, a fraction of the probe beam power also passesthrough splitter 24 and falls on an incident power detector 30. As iswell known to those skilled in the art, incident power detector 30 isprovided to monitor fluctuations in the output power of the probe beamlight source. As discussed in U.S. Pat. No. 5,181,080, the incidentpower detector can be modified to minimize measurement errors whicharise due to asymmetries of the beam.

Light reflected from the surface of the sample passes up throughsplitter 24 towards photodetector 40. Prior to reaching detector 40, thebeam 22 is passed through a quarter-wave plate 42 for retarding thephase of one of the polarization states of the beam by 90 degrees. Itshould be noted that the quarter-wave plate could be located in the beampath prior to the probe beam striking the sample so that the systemwould operate with circularly polarized light. The latter approach mighthave some advantages in reducing the aberrations created by lens 26. Inaddition, while a phase retardation of 90 degrees will maximize thedesired signal, other intermediate levels of retardation would bepossible.

The beam is then passed through a linear polarizer 44 which functions tocause the two polarization states of the beam to interfere with eachother. In order to maximize the desired signal, the axis of thepolarizer should be oriented at an angle of 45 degrees with respect tothe fast and slow axes of the quarter-wave plate 42.

In accordance with the subject invention, detector 40 is configured togenerate independent signals from regions along two pairs of mutuallyorthogonal axes. In this first embodiment, this goal is achieved byusing a photodetector having eight pie shaped segments. As illustratedin FIG. 3, the detector surface includes eight, radially disposedsegments 1–8. Each segment will generate an output signal proportionalto the magnitude of the power of probe beam striking that quadrant. Thissignal represents an integration of the intensities of all the rayshaving different angles of incidence with respect to the sample surface.While this integration approach results in the loss of some informationcontent as compared to an analysis of individual rays, the compositeapproach does provide significantly greater sensitivity through enhancedsignal to noise performance.

The probe beam 22 should be centered on the detector 40 so that eachsegment intercepts an equal portion of the probe beam. The probe beamshould underfill the detector.

The output of the segments is supplied to the processor 50 forevaluation. As in the prior art, the outputs of all the segments can besummed to provide a measure of the full power of the reflected beam. Asdiscussed below, this full power measurement can be used as an input toa regression analysis to determine the characteristics of the sample.

In accordance with the invention herein, the processor can also generatemeasurements which allow additional ellipsometric information to bederived as compared to the prior art approach. This difference can bestbe understood by comparing the two approaches.

In general, one measures the total reflectivity of the sample inaccordance with the following equation:R=½(|r _(p)|²⁺ |r _(s)|²)

The sine of the ellipsometric phase shift δ is determined by theequation:tan ψe ^(iδ) =|r _(p) /r _(s) |e ^(iδ)

With the prior art detector of FIG. 1, the information is derived asfollows:Σ==1+2+3+4Δ=(1+3)−(2+4)R=Σ/Σ_(o) and sin δ=π/2 (Δ/|r _(p) r _(s)|)

where Σ_(o) is the measured sum signal from a known reference material.

The complete determination of the polarization state requires ameasurement that gives tan ψ and that requires an additionalmodification to the detector as shown in FIG. 3. As noted above, the sumis formed from the outputs of the eight segments as follows:Σ=1+2+3+4+5+6+7+8

Then as previously:R=Σ/Σ_(o) and withΔ₁≡(1+2+5+6)−(3+4+7+8)Δ₂≡(2+3+6+7)−(4+5+8+1)

one hasΔ₁/Σ=4/π(tanψ/tan² ψ+1)sin δ andΔ₂/Σ=−2/π(tan² ψ−1/tan² ψ+1)

As can be seen, the information from the eight segments can be used toderive both Ψ and δ. In practice, those quantities may not be needed. Infact, it is often preferable to use the measurements more directly inthe evaluation of sample parameters. Thus, the invention should notconsidered limited to determining both Ψ and δ, but rather is anapproach which provides an additional measurement for analyzing thesample.

Although the measurements made by the subject apparatus could be used bythemselves to characterize a sample, those measurements can also becombined with other measurement obtained from additional opticalmetrology devices in manner discussed in U.S. Pat. No. 6,278,519. Asystem with multiple inspection technologies generates a number ofindependent measurements which are then combined in a regressionanalysis to determine sample parameters.

Combining data from multiple devices is a procedure quite well known andneed not be described in detail. In brief, a mathematical model isdefined which describes the structure under test. A best guess of sampleparameters is assigned to the model and the optical response iscalculated. The calculated optical response is compared to the measuredoptical response. Any deviations between the calculated optical responseand the measured optical response are used to vary the initial startingparameter guesses and the process is repeated in an iterative fashionuntil satisfactory convergence is reached. (See, for example, the Leng,article cited above.) As noted above, with such an analytical approachit is not necessary to actually calculate Ψ and δ. Rather the inputsfrom Δ₁ and Δ₂ calculations as set forth above (as well as the fullpower measurement) are used as inputs actual measurements to the fittingalgorithm. Of course, if desired, the Δ₁ and Δ₂ calculations can also beused to calculate Ψ and δ if desired.

These types of analyses are suitable for both thin films and physicalstructures formed on the sample. It is also possible to use a databaseor library type approach where a set of the optical responses of aparameterized sample are calculated in advance and stored. The measuredresponse is compared to the stored responses to determine sampleparameters. (See, for example, US Published Applications 2002/0038186and 2002/0035455). The subject invention is not intended to be limitedeither by the type of sample being measured, nor the specific algorithmsused to analyze the data.

As will be discussed below, there are a number of alternative detectorconfiguration which can be used to generate the information of interest.One common thread is that measurements are taken along a first pair oforthogonal axes and along a second pair of orthogonal axes, with thesecond pair being perpendicular to the first pair.

FIG. 4 illustrates an alternative configuration for such a detector 440that satisfies these criteria. Each segment 1–8 is a linear detectorarranged in a star-shaped configuration that corresponds to the pieshaped segments of FIG. 3. The analysis of the measurements discussedabove with respect to FIG. 3 would be identical to that of FIG. 4.

Those skilled in the art will also appreciate that segments 5 to 8 foreither the FIG. 3 or FIG. 4 embodiment are complimentary to segments 1to 4 so that a detector with only segments 1 to 4 might be used.However, adding the outputs of segments 5 to 8 to the outputs ofsegments 1 to 4, respectively, makes the detector insensitive to smallshifts in the probe beam spot positioning and is therefore the preferredapproach.

The detector configurations of FIGS. 3 and 4 will produce a measurementthat represents an average over all the incident angles. To examine anarrower range of angles of incidence, a detector 540 as shown in FIG. 5with eight segments arranged in an annulus might be used. Such adetector might be of interest where the probe beam is directedsubstantially normal to the sample before focusing as shown in FIG. 2.In such a case, the radially outermost rays of the probe beam have thehighest angles of incidence. The annular ring configuration of FIG. 5would capture only those higher angle of incidence rays, which, in thecase of isotropic samples, often carry the most information.

The subject invention could also be implemented using a detector 640comprised of a two dimensional array of detector elements or pixels asshown in FIG. 6A. Such a detector could be defined by a CCD array. Thepixels in the array could be mapped into eight segments to correspond tothe detectors shown in FIGS. 3 to 5. The mapping of pixels to thedetector segments of FIG. 3 is shown in FIG. 6B. The processor wouldselect the output from the appropriate pixels to calculate Δ₁ and Δ₂ andderive the ellipsometric information.

With a sufficiently dense array almost any configuration of angles ofincidence is possible. This is especially significant for measuringphysical structures (CD) where the xy orientation of the samplestructure and the orientation of probing radiation is relevant andsignificant. With a system such as described above it is possible (withminimal moving mechanisms) to probe the CD structure from all angles ofincidence, planes of incidence and polarizations relative to theorientation of the CD structure.

If it is desired to extend this concept to measure multiple wavelengths,the laser light source 20 could be a white light source that wouldgenerate a polychromatic probe beam. A wavelength selective filter 60(shown in phantom line in FIG. 1) would then be placed somewhere in thelight path between the light source and the detector. The filter couldtake the form of simple band pass (color) filters which are selectivelymoved into the path of the beam. Alternatively, a monochrometer could beused to sequentially select narrow wavelength regions. Of course atunable laser or multiple lasers with different wavelengths could alsobe used.

FIGS. 7 and 8 illustrate two further embodiments wherein detectors withonly four quadrants can be used to obtain the measurements requiredherein. FIG. 7 a illustrates a quadrant detector 740 positioned to takea first set of measurements. Segment 1 would generate an outputequivalent to segments 1 and 2 of the detector of FIG. 3. Similarly,segment 2 of the detector of FIG. 7 a would generate an outputequivalent to segments 3 and 4 of the detector of FIG. 3, segment 3 ofthe detector of FIG. 7 a would generate an output equivalent to segments5 and 6 of the detector of FIG. 3 and segment 4 of the detector of FIG.7 a would generate an output equivalent to segments 7 and 8 of thedetector of FIG. 3. This measurement would permit the calculation of Δ₁.

Once this measurement is made, the quadrant detector could be rotated 45degrees to a position shown in FIG. 7 b. In this orientation, segment 1of the detector of FIG. 7 b would generate an output equivalent tosegments 2 and 3 of the detector of FIG. 3. Similarly, segment 2 of thedetector of FIG. 7 b would generate an output equivalent to segments 4and 5 of the detector of FIG. 3, segment 3 of the detector of FIG. 7 bwould generate an output equivalent to segments 6 and 7 of the detectorof FIG. 3 and segment 4 of the detector of FIG. 7 b would generate anoutput equivalent to segments 8 and 1 of the detector of FIG. 3. Thismeasurement would permit a calculation of 66 ₂.

The detector configuration of FIG. 7 might be desirable to simplify andminimize the cost of the detector. However, the trade off would be thatthe measurement would require two steps and the rotation of an element.Rather than rotating the detector, similar results might be achieved ifeither the polarizer or analyzer or both were rotated to provideindependent measurements.

FIG. 8 illustrates a configuration conceptually similar to FIG. 7 butwhich would allow both measurements to be taken at once. Morespecifically, after the probe beam is reflected from the sample andpasses the polarizer and waveplate, it can be divided by beam splitter802. Two separate quadrant detectors 840 and 842 are each located in oneof the two beam paths. The quadrants of detector 840 are offset from thequadrants of detector 842 by 45 degrees. The output of the quadrants ofdetector 840 can be used calculate Δ₁ and the output of the quadrants ofdetector 842 can be used to calculate Δ₂. As an alternative, one couldplace the beam splitter in the path of the reflected probe but beforethe quarter waveplate and polarizer. In this alternative, it would benecessary to place a quarter waveplate and a polarizer in each path. Inthis arrangement, it would also be possible to orient both detectors atthe same azimuthal angle, but have different positions for either thewaveplate or the polarizer in one path as compared to the other path.

While the subject invention has been described with reference to apreferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

1. An apparatus for evaluating the characteristics of sample comprising:a light source for generating a probe beam; a focusing element forfocusing the beam onto the surface of the sample in manner to create aspread of angles of incidence with varying azimuthal angles; a retarderlocated in the path of the beam; a polarizer located in the path of thereflected beam; a detector in the path of the reflected beam, saiddetector defined by a two-dimensional array of detector elements, eachelement generating output signals; and a processor for correlating theindividual output signals from the detector elements to specific anglesof incidence and a plurality azimuthal positions, said azimuthalpositions including at least three individual planes of incidence withrespect to the sample, said processor evaluating the characteristics ofthe sample based on said correlation.
 2. An apparatus as recited inclaim 1, wherein said processor analyzes the signals corresponding tofour planes of incidence, a first pair being orthogonal to each otherand the second pair located in planes bisecting the first pair.
 3. Anapparatus as recited in claim 1, wherein said light source is a narrowband laser.
 4. An apparatus as recited in claim 1, wherein the lightsource generates a polychromatic probe beam and further including ameans to permit detection of the probe beam as a function of wavelength.5. An apparatus as recited in claim 4, wherein said means is defined bya selectable color filter located in the path of the beam.
 6. Anapparatus for evaluating the characteristics of sample comprising: alight source for generating a probe beam; a focusing element forfocusing the beam onto the surface of the sample in manner to create aspread of angles of incidence with varying azimuthal angles; a detectorin the path of the reflected beam, said detector defined by atwo-dimensional array of detector elements, each element generatingoutput signals; and a processor for correlating the individual outputsignals from the detector elements to specific angles of incidence and aplurality azimuthal positions, said azimuthal positions including atleast three individual planes of incidence with respect to the sample,said processor evaluating the characteristics of the sample based onsaid correlation.
 7. An apparatus as recited in claim 6, wherein saidprocessor analyzes the signals corresponding to four planes ofincidence, a first pair being orthogonal to each other and the secondpair located in planes bisecting the first pair.
 8. An apparatus asrecited in claim 6, wherein said light source is a narrow band laser. 9.An apparatus as recited in claim 8, wherein said means is defined by aselectable color filter located in the path of the beam.
 10. Anapparatus as recited in claim 6, wherein the light source generates apolychromatic probe beam and further including a means to permitdetection of the probe beam as a function of wavelength.